Fuel cells have been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. One example of a fuel cell is the Proton Exchange Membrane (PEM) fuel cell. The PEM fuel cell includes a membrane-electrode-assembly (MEA) that generally comprises a thin, solid polymer membrane-electrolyte having a catalyst and an electrode on both faces of the membrane-electrolyte.
The MEA generally comprises porous conductive materials, also known as gas diffusion media, which distribute reactants over the surfaces of the electrode layers. Fuel, such as hydrogen gas, is introduced at the anode where it reacts electrochemically in the presence of the catalyst to produce electrons and hydrogen cations. The electrons are conducted from the anode to the cathode through an electrical circuit disposed therebetween. Simultaneously, the hydrogen cations pass through the electrolyte to the cathode where an oxidant, such as oxygen or air, reacts electrochemically in the presence of the electrolyte and catalyst to produce oxygen anions. The oxygen anions react with the hydrogen cations to form water as a reaction product.
The MEA is typically interposed between a pair of electrically conductive contact elements or bipolar plates to complete a single PEM fuel cell. Bipolar plates serve as current collectors for the anode and cathode, and have appropriate flow channels and openings formed therein for distributing the fuel cell's gaseous reactants (i.e., the H2 & O2/air) over the surfaces of the respective electrodes. The bipolar plates may serve several additional purposes, such as provide mechanical support to withstand the compressive forces applied to hold the fuel cell stack together and provide a means to remove excess heat generated by the exothermic fuel cell reactions occurring in the fuel cell, for example.
An important measure of the fuel cell is its volumetric power density. High volumetric power density is desirable for vehicle applications of fuel cells. Volumetric power density is measured as the watt density per cm2 of an individual fuel cell times the quantity of cells per linear centimeter of stack height. Therefore, it is desirable to design thin cells to achieve high volumetric power density. Volumetric power density is mostly a function of the physical design of the fuel cell components and the design of the bipolar plates. The design of the bipolar plates in the prior art has been driven by many wide ranging factors, such as cell chemistry, reactant flow configurations, material selection, system pressurization, operating temperature, and system cooling requirements, for example.
Accordingly, a need exists to produce a bipolar plate which minimizes mass transport resistance at high current density, particularly on the cathode side of the fuel cell. Desirably, the bipolar plate also achieves a minimized material cost, an optimized use of coolant, and simpler manufacturability, while maximizing volumetric power density.